1. Field of the Invention
This invention relates to a magnetic disk unit, and more particularly to a magnetic disk unit in which a floating magnetic head is urged against the magnetic disk surface or drawn away therefrom.
2. Description of the Prior Art
Heretofore, there has been employed, as a magnetic head for a magnetic disk unit, a dynamic pressure floating type head in which a magnetic head incorporated in a slider floats off a recording medium by utilizing the dynamic effect of an air current resulting from revolution of the magnetic recording medium. To increase the recording density of the magnetic disk unit, it is necessary to minimize the floating space between the magnetic recording medium and the magnetic head. To this end, efforts are now being made to provide for miniaturized floating space. In order to maintain the floating space constant against axial run-out and vibration of the magnetic recording medium, a load force (a pressure) in equilibrium with the air pressure generated in the floating space is applied to the floating magnetic head by a load spring. Accordingly, at the start and end of the operation, no floating force is produced and the magnetic head makes contact with the magnetic disk surface.
The floating space is dependent upon the floating surface configuration of the slider and the pressure applied to the magnetic head. In the case of a conventional cylindrical surface slider whose floating surface is cylindrical, a pressure of 350 g is applied to the magnetic head for obtaining a floating space of 1.1 .mu.m during steady-state floating (the speed of the recording medium being 40 m/sec.). Where the magnetic head and the magnetic disk surface slide on each other under the pressure of 350 g every time the operation is started and stopped, the magnetic head and the magnetic disk surface are seriously damaged, making this method entirely impracticable. The so-called loading on-off system, in which the pressure to the floating magnetic head is removed when the operation is started and stopped, has been used. With such a system, at the start of operation, after the magnetic disk reaches a predetermined rotating speed a force sufficient to float the slider against the aforesaid pressure develops, and the magnetic head is pressed against the magnetic disk surface, i.e. loaded on the magnetic disk surface. And when the operation is stopped, the magnetic head is lifted off the magnetic disk surface to remove the pressure and then the magnetic disk is stopped.
In an example of the conventional loading on-off system, the loading on-off operation is achieved by the movement of a carriage of the magnetic head. In this case, since the moving (seeking) speed of the carriage is set above a certain value so as to immediately respond to a write-read request, the floating magnetic head approaches the magnetic disk surface at an appreciably high speed when loading on the latter. Accordinly, unless the floating surface of the magnetic head and the magnetic disk are set and adjusted with extremely high accuracy, a floating force large enough to counteract the kinetic energy which is produced when the floating magnetic head approaches the magnetic disk surface is not generated, permitting the magnetic head to get into contact with the magnetic disk. When the magnetic head loads on the magnetic disk surface, the magnetic disk is rotating at a high speed, so that the loading of the magnetic head on the magnetic disk inflicts serious damage on both of them. (So-called head crush occurs.) Since the moving (seeking) speed is required to be 22 cm/sec., the range in which head crush does not occur, in the case of the cylindrical-surfaced slider, is within about .+-.10' in its pitch direction and within about .+-.15' in its roll direction, and extremely high accuracy is needed in the setting of the posture of the slider.
For further miniaturization of the floating space in this conventional system, it is necessary to increase the aforesaid pressure substantially in inverse proportion to the square of the floating space or reduce the floating surface of the slider. But this leads to the reduction of the range in which head cruch does not occur, and introduces difficulty in the setting and adjustment of the slider.
To avoid such a defect, there has recently been put in use the so-called contact start-stop system which does not employ the loading on-off operation. With this system, the area of the floating surface is reduced to decrease the required floating force and the force balancing with the floating force is reduced to approximately 10 g. A magnetic head slider is used which is adapted so that even when the floating surface and the magnetic disk surface are in sliding contact with each other at the start and stop of the operation, neither surface is rapidly worn, and the operation is started and stopped with these surfaces held in contact with each other.
In this known contact start-stop system, the magnetic head easily floats off the magnetic disk surface at the start of the operation and, since a flat-surfaced slider is used, the surface pressure on the magnetic disk surface is low to prevent wear of the both surfaces. Further, the slider has a large stiffness in its pitching direction and presents a stable floating characteristic. Since this system does not involve the mechanism for the loading on-off operation and does not require such high accuracy in the setting and adjustment of the floating surface, it is advantageous as compared with the landing on-off system but has the following defects.
That is, where the floating space is further reduced so as to obtain enhanced recording density, there occurs the dew condensation phenomenon in which dew drops are precipitated on the floating surface, as described in detail later on. The dew drops do not readily scatter, so that in the case of stopping the operation (the contact stop) with dew drops remaining on the floating surface, the floating magnetic head and the magnetic disk surface adhere to each other due to the surface tension of the dew drops and do not readily separate from each other. In the above example, a force larger than 100 g is needle for separating them from each other. Accordingly, if the operation is started with the floating magnetic head and the magnetic disk surface sticking to each other, an accident occurs such as breakage of the joint between the floating magnetic head and gimbal, destruction of the gimbal itself, etc.
For achieving a very small floating space, the magnetic disk surface and the floating surface of the floating magnetic head must to be finished with such high accuracy that their surface roughness is less than one fifth or sixth of the floating space. Leaving such accurately finished surfaces in close contact with each other, dew drops are also condensed between them. Accordingly, where the magnetic disk unit is not used for a long time, the same trouble as mentioned above also occurs.